Dielectric drying systems are known and are currently in use or have been proposed for use in agriculture, polymer manufacture, pharmaceuticals, bulk powder, food processing, wood products, and other industries. One of the key industries using these dielectric drying systems is the wood products industry and the present invention will be described particularly with respect to the wood products industry although the invention, with suitable modifications where required, may be applied in the other industries in which dielectric drying is to be performed.
In dielectric drying systems (particularly those for drying wood of the type described in U.S. Pat. No. 3,968,268 issued Oct. 19, 1976 to Koppelman), it is conventional practice for the lumber to be moved into the drying chamber, at least one power electrode that will emit electromagnetic energy to a grounding electrode to complete the circuit is positioned near or in contact with the load. After the load has been positioned in the kiln, the kiln chamber is closed and the drying process is commenced by applying a negative pressure in the chamber and applying RF power (energy) to the load through the power electrode(s). In the arrangement shown in this patent, two vacuum draw headers are provided through the top of the kiln with one adjacent to each longitudinal end of the kiln.
WO 99/18401 published Apr. 15, 1999 inventor Wolf discloses a kiln with vertical electrodes similar to Koppelman and once Wolf draws the chamber down to the operating pressure used for drying, he applies vacuum at a single point on the side of the load positioned behind one of the electrodes.
Japanese patent JP4121578 published Apr. 22 1992 inventor Nishihama also discloses the use of vacuum connections one through the roof and a second through the floor, the one through the floor is referred to as sucking a part of the drain from the bottom.
In dielectric drying systems of which the Applicants are aware in the wood products industry, vacuum is drawn from one or two vacuum headers, positioned on top of the chamber (see for example the vacuum headers described in the above Koppelman patent). The belief in the industry prior to the present invention was that vacuum draw location from the chamber was irrelevant to the operation of dielectric vacuum drying processes such as radio frequency vacuum drying (RFVD).